Wireless communication techniques are applied not only to the field of information communication but also to the field of logistics management, manufacturing, and the like. An IC tag for wireless communication (hereinafter, simply referred to as an “IC tag” or a “tag”) is widely known as a product that plays an important role in RFID (radio frequency identification) technology. Since an IC tag can be used in a wide range of applications for logistics management or low-cost information storage media, the IC tag is used in various environments.
An IC tag is a wireless communication apparatus including a chip that stores data such as an identification number and an antenna that is used for transmitting and receiving radio waves, and is significantly advantageous in that the IC tag can be realized as a small, thin, and light apparatus.
In order to sufficiently make use of such an advantage, it is preferable that there is no limitation on the position to which the IC tag is stuck, and that the IC tag is configured so as to be capable of performing communication regardless of the position or manner in which the IC tag is stuck.
FIG. 30 is a cross-sectional view schematically showing an IC tag 1 according to a conventional technique. An RFID (radio frequency identification) system is a system used for automatically recognizing individual objects, and basically includes a reader and a transponder. The IC tag 1 is used as a transponder of this RFID system.
The communication system in FIG. 30 is an electromagnetic induction type system that transfers energy or signals through the coupling of magnetic fluxes between coil antennas of a tag and a reader. A passive IC tag of the electromagnetic induction type has a maximum communication distance of up to approximately 1 m, and is used for short range communication. The frequency used in this system is, for example, in an LF (low-frequency) band or an HF (high-frequency) band. The IC tag 1 has a coil antenna 2 that is a magnetic field type antenna for detecting magnetic force lines, and an integrated circuit (IC chip) 3 that performs wireless communication using the coil antenna 2. A configuration is adopted in which the IC tag 1 transmits information stored in the IC chip 3 upon receiving a request signal from a reader 5, in other words, the reader can read information held in the IC tag 1. The IC tag 1 is stuck to a merchandise product or the like, and used for managing merchandise products, for example, for preventing merchandise products from being stolen or for keeping track of inventory status.
In a case where a communication-jamming member 4 (an object made of a conductive material in this example) is present near the IC tag 1 (e.g., the IC tag 1 is stuck to a metal merchandise product in use), when the magnetic force lines of the magnetic field formed by electromagnetic wave signals transmitted and received by the IC tag 1 approach the communication-jamming member 4, the magnetic force lines pass through a position near the surface of the communication-jamming member 4 in parallel thereto without entering the communication-jamming member 4, unlike the behavior in the case of an electric field. As a result, an eddy current is generated on the surface of the communication-jamming member 4, and the electromagnetic wave energy is converted into thermal energy and absorbed (resistive loss). When the energy is absorbed in this manner, electromagnetic wave signals are attenuated, and the IC tag 1 cannot perform wireless communication. Furthermore, a phenomenon occurs in which an induced eddy current generates a magnetic field in an opposite direction to the magnetic field for communication of the tag (diamagnetic field), thereby canceling the magnetic field. This phenomenon also makes it impossible for the IC tag 1 to perform wireless communication. Moreover, for example, there is a phenomenon in which the influence of the communication-jamming member 4 shifts the resonant frequency of the IC tag 1. Accordingly, the electromagnetic induction type IC tag 1 cannot be used near the communication-jamming member 4.
As described above, “communication-jamming member” is a collective term for materials that, when present near an antenna, change the resonant frequency of an antenna designed for a free space environment, or reduce the exchange of electromagnetic waves between antennas. In the invention, countermeasures are taken mainly for the case in which the communication-jamming member is made of metal.
FIG. 31 is a simplified cross-sectional view showing an IC tag 1A according to another conventional technique. The IC tag 1A shown in FIG. 31 is similar to the IC tag 1 in FIG. 30, and thus, the corresponding constituent elements are denoted by the same numerals, and only different constituent elements in the configuration will be described. In order to solve the problem of the IC tag 1 in FIG. 30, the IC tag 1A in FIG. 31 is configured to include a magnetic wave absorbing plate 7 disposed between the antenna 2 and the member 4 that is a product to which the IC tag 1A is attached. The magnetic wave absorbing plate 7, which is a sheet having a complex relative magnetic permeability, is made of a highly magnetically permeable material such as sendust, ferrite, or carbonyl iron, that is, a material having a high complex relative magnetic permeability.
The complex relative permeability has a real part and an imaginary part. As the real part increases, the complex relative permeability increases. In other words, a material having a high complex relative permeability has a large real part in that complex relative permeability. In the case where a material having a large real part in the complex relative permeability thereof is present in a magnetic field, the magnetic force lines are collected and passed through the material. In the IC tag 1A using a magnetic field type antenna 2 that detects magnetic force lines in electromagnetic induction-type communication, a magnetic absorbing plate 7 is provided so as to prevent leakage of the magnetic field toward the communication-jamming member 4, and thus, even when the IC tag 1A is used near the communication-jamming member 4, attenuation of the magnetic field energy is suppressed, and wireless communication can be performed. This sort of IC tag 1A is disclosed, for example, in Japanese Unexamined Patent Publication JP-A 2000-113142. Here, the wireless communication is performed based on a modulated magnetic field, and this communication is electromagnetic induction-type communication according to the invention.
In this communication-improving method, the magnetic absorbing member 7 having magnetism is disposed between the first antenna 2 and the communication-jamming member 4, and thus, magnetic force lines used for communication are absorbed by and passed through the magnetic absorbing member 7. The permeability of the magnetic absorbing member 7 is an important factor. This improving method is effective in the case of electromagnetic induction-type communication that performs communication through magnetic coupling, but is not effective in the case of radio-wave-type communication. The reason for this is that, although the travel direction of the magnetic field (magnetic flux loop) can be controlled by a magnetic material even in a near field, high-frequency radio waves used in radio-wave-type communication have a strong tendency to proceed in straight lines, and the travel direction cannot be easily changed without an antenna member or the like.
The communication mechanism of the IC tag varies depending on the frequency of radio waves used. In a case where radio waves having a higher frequency are used, radio-wave-type communication is adopted in which energy and signals are transferred by exchanging electromagnetic waves between an antenna of a tag and an antenna of a reader. For example, in the case where radio waves in a UHF (ultrahigh frequency wave) band, an SHF (centimeter wave) band, or an EHF (millimeter wave) band are used, a tag transmits and receives information according to radio-wave-type communication using an electric-field-type antenna such as a so-called dipole antenna. Unlike the case of the electromagnetic induction-type communication using the coupling of magnetic fluxes, radio-wave-type communication realizes long range communication by radiating electromagnetic waves into the air. The wireless communication-improving sheet member, the wireless IC tag, the antenna, and the wireless communication system using the same of the invention are applied to this wireless radio-wave-type communication. The electromagnetic induction type and the radio wave type are different from each other in the relationship between the wavelength of electromagnetic waves and the distance between antennas. In the case where the distance is short for the wavelength, an electromagnetic induction type is used in which a change in the electric field/magnetic field is transmitted to the other antenna before being radiated into the air. In the case where the distance is long for the wavelength, a radio wave type is used in which information is transmitted as electromagnetic waves through the air. Furthermore, antennas used in the electromagnetic induction type are magnetic-field-type antennas such as a coil antenna, and those used in the radio wave type are electric-field-type antennas such as a dipole antenna or a patch antenna. That is to say, the communication system itself is different therebetween.
Also in the case where a conductive material (a communication-jamming member), such as metal, is present near an IC tag that performs radio-wave-type communication, communication becomes impossible through a mechanism different from that of the electromagnetic induction type. When reception by the IC tag causes a resonant current to flow through an antenna, a current in the opposite direction is induced on the side of a nearby metal face, and the induced current significantly lowers the impedance of the transferred signals. Accordingly, the impedance does not match the input impedance of an IC chip designed for communication in free space, and the possible communication distance is shortened.
Typically, an electric-field-type antenna such as a dipole antenna, a monopole antenna, and a loop antenna is designed as follows: When the antenna receives radio waves having a specific frequency, a resonant current is formed at the antenna, and when this current flows through an IC chip, the impedance matches the input impedance of the chip.
FIG. 32 is a cross-sectional view showing a momentary electric field (the direction of a current) formed near a tag main body 22 (IC tag) in a state where the tag main body 22 (IC tag) is disposed near a conductive member, which is a communication-jamming member.
In the case where a communication-jamming member 212 is present near an antenna element 211, a resonant current I11 is formed that is directed to one end portion (hereinafter, referred to as a “first end portion”) 211a from the other end portion (hereinafter, referred to as a “second end portion”) 211b of the antenna element 211, and a current I12 is formed that is directed from one portion (hereinafter, referred to as a “first portion”) 212a to the other portion (hereinafter, referred to as a “second portion”) 212b on the communication-jamming member 212, and thus, currents in the opposite directions flow at the antenna element 211 and the communication-jamming member 212. That is to say, an antenna having the same scale and that performs the opposite operation is formed on the communication-jamming member 212.
The voltage that is applied to an IC 217 or that is applied from the IC 217 started by the transmission of energy is an alternating voltage, and thus, a state in which a current in the direction shown in FIG. 32 is formed and a state in which a current in the opposite direction is formed alternately occur. When the frequency increases, this state is equivalent to a state in which a current I0 is formed between the first end portion 211a of the antenna element 211 and the first portion 212a of the communication-jamming member 212, and between the second end portion 211b of the antenna element 211 and the second portion 212b of the communication-jamming member 212, that is, a short circuit occurs, at a high frequency, between the first end portion 211a of the antenna element 211 and the first portion 212a of the communication-jamming member 212, and between the second end portion 211b of the antenna element 211 and the second portion 212b of the communication-jamming member 212. When such a short circuit occurs at a high frequency, a closed circuit is formed by the antenna element 211 and the communication-jamming member 212, and the current value increases compared with the case in which the communication-jamming member 212 is not present nearby. That is to say, compared with the case in which the communication-jamming member 212 is not present near the antenna element 211, the impedance is significantly lowered. As a result, the impedance does not match the input impedance of the chip, and information signals are not transferred. Accordingly, the possible communication distance is shortened.
Furthermore, not only metal but also paper, glass, resin, liquid, magnetic materials, other antennas, or the like may cause the communication properties of the IC tag to deteriorate when the material is present nearby.
In the case of these materials, the permittivity or the permeability of the materials changes the resonant frequency of the antenna, the resonant frequency of the antenna becomes different from the frequency of radio waves used by a component with which communication is performed, and the possible communication distance is shortened.
FIG. 33 is a cross-sectional view schematically showing an IC tag 1B according to still another conventional technique. The IC tag 1B shown in FIG. 33 is similar to the IC tag 1 in FIG. 30, and thus, corresponding constituent elements are denoted by the same reference numerals, and only different constituent elements will be described. In the IC tag 1B in FIG. 33, a first antenna 2 that is a dipole antenna and an IC 3 are arranged on a substrate 8, and a second antenna 1C is disposed via a first spacer 9 on a communication direction side of the first antenna 2. Furthermore, in the IC tag 1B, a second spacer 11 is disposed on the substrate 8 on the opposite side to the first antenna 2. The IC tag 1B is used near the communication-jamming member 4 in a state where the substrate 8 and the second spacer 11 are interposed between the first antenna 2 and the communication-jamming member 4. The IC tag 1B has a configuration in which the second antenna 1C that is an auxiliary antenna is disposed on the communication direction side of the first antenna 2, to which an IC is connected, and thus, the intensity of radio waves of the first antenna 2 is prevented from being attenuated by the communication-jamming member 4. This sort of IC tag 1B is disclosed, for example, in Japanese Unexamined Patent Publication JP-A 2005-210676.
The IC tag 1B in FIG. 33 is a specially designed tag having an integrated structure in which the first antenna 2 is held between the second antenna 1C, and the first and the second spacers 9 and 11. With this structure, the versatility to improve communication performance by simply tackinessly or adhesively applying a commercially available wireless IC tag is not provided. Furthermore, even after such countermeasures are taken, the first antenna 2 and the IC 3 are positioned near the communication-jamming member 4, and the influence of the type of the communication-jamming member 4, for example, a difference in permittivity, shifts the resonant frequency.